Machines have been developed for forming a tensioned loop of thermoplastic strap around an object. Such machines typically include means for feeding strap which is automatically or manually formed into a loop about the object, means for pulling the strap loop trailing portion to tension the strap loop about the object, means for securing the overlapping strap portions together by melting and resolidifying regions of the strap, and means for severing the strap trailing portion from the loop.
A number of methods and apparatus have been developed over the years for feeding and then tensioning the strap in such machines. See, for example, the disclosures in U.S. Pat. Nos. 4,011,807 and 3,420,158 and see the following commercial automatic strapping machines (1) the "SUPERSTRAP M" machine sold in the U.S.A. by Nakano Bussan Company, 45-5, KAMATA 5-CHOME, OHATA-ku, Tokyo, 144, Japan, (2) the "MODEL MS POWER STRAPPING MACHINE" manufactured and sold in the U.S.A. by Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025, U.S.A., and (3) the "MCD 700/300" machine manufactured and sold in the U.S.A. by Signode Corporation, 3600 West Lake Avenue, Glenview, Ill. 60025, U.S.A. Such conventional tensioning and feeding assembly designs include a feed wheel for feeding the strap forward to form the loop and a retraction or tensioning wheel for pulling the loop tight about the object. In addition, the assembly disclosed in the U.S. Pat. No. 4,011,807 includes a rotatable winder drum with a pair of arcuate feed guide means which cause the strap to be wrapped about the drum when it is rotated so as to apply high tension to the strap loop.
Although the various conventional strap feeding and tensioning assembly designs work well for the applications for which they are intended, it would be desirable to provide an improved strap feeding and tensioning assembly which could accommodate a variety of strap feed rates and tensioning rates without "milling" or otherwise damaging the strap.
In some applications, it has been found that conventional strap feeding and tensioning assemblies do not easily accommodate an obstruction in the strap feed path. When the end of the strap being fed encounters an obstruction, it can buckle and crinkle before the machine is shut off. This can damage the strap and may thus require a new length of strap to be fed into the machine. It would be beneficial if an improved strap feeding and tensioning assembly could readily accommodate obstructions in the strap feeding path in a manner so as to prevent the strap end from being urged against the obstruction with an excessive amount of force that could cause the strap to buckle and crinkle.
In machines employing conventional high speed tensioning mechanisms to initially constrict the loop of strap about the object, the object is subjected to an initial high load when the constricting loop first contacts the package. This is because the initially formed, untensioned loop offers very little resistance to retraction of the strap until the loop is small enough to contact the periphery of the object. At that point, both the object and the strap are typically subjected to a substantially increased force as the tensioning mechanism continues to retract the strap. It would be desirable to provide an improved strap feeding and tensioning method and apparatus wherein the tension could be applied in such as way so as to reduce such initial transient impact loading (e.g., loading spikes).
Also, it would be advantageous to provide an improved strap feeding and tensioning method and apparatus of the type described wherein a relatively high tension could be applied to the strap loop after the strap loop has been initially drawn tight around the object. It would be desirable to provide such a high tension capability in a manner that would permit the use of a relatively simple high tensioning mechanism.
Finally, it would be desirable to provide the improved strap feeding and tensioning method and apparatus of the type described with the capability for very rapidly feeding the strap, rapidly withdrawing the strap tight about the object, and rapidly applying high tension to the object.